1. Field of Invention
The invention relates to the field of metal forming, and particularly to an improved method of removing a formed part from a die cavity.
2. Background of Prior Art
Removal of formed parts from a die can be difficult, particularly if the part is formed at high temperatures and is to be removed from the die while still hot. For example, when superplastically forming titanium parts, as disclosed in U.S. Pat. No. 4,181,000, Method for Superplastic Forming by Hamilton, et al., the forming temperatures are around 1600.degree. F. During superplastic forming, the preform is clamped between two dies and stretched into the form of the cavity in one of the dies. Considerable friction forces develop and after forming it is often difficult to remove the part. A typical procedure is to use a crowbar, or the like, to pry the formed part out of the die. This can create problems because if the part is still at high temperatures, and thus possessing little resistance to deformation, the localized loading may warp the part making it unusable. This is especially true with large parts that have been formed in dies having near to zero cavity draft angles. On the other hand, if the part is allowed to cool, so will the die, making the production rate exceedingly low and, additionally, wasting a lot of energy reheating the die. Additionally, differential shrinkage between the part and die can lead to destruction or the "locking" of the part therein.
The other most commonly used method is to mechanically eject. For example, U.S. Pat. No. 3,535,766, Machine Assembly Method by L. Jymes, and U.S. Pat. No. 3,642,415, Plunger-and-Diaphragm Plastic Sheet Forming Apparatus by H. G. Johnson and U.S. Pat. No. 3,587,144, Multi-Sections Mold with Air Pressure Ejecting Ring by E. H. Mechling use pneumatically driven plungers to eject the formed part from the die.
The problem with mechanical ejection by use of pins or the like is that it also creates localized loading which may deform the part, again, particularly if the part is at high temperature. There is also a problem of maintaining the pin flush with the cavity wall so that no localized discontinuities are created on the part.
One of the most common methods of ejecting a formed part from a die cavity is to use pneumatic pressure. Typically, one or more ports are provided in the die cavity so that air can be injected between the part and cavity to eject the part. Examples of pneumatic ejection systems can be found in U.S. Pat. No. 3,952,991, Apparatus for Ejecting a Workpiece from a Mold Cavity by Schneider; U.S. Pat. No. 3,556,650, Diaphragm-Type Sheet Forming Method by H. G. Johnson. With the use of pneumatic ejection, localized loading may also occur but is less prevalent. Having a port in the cavity wall may also create unacceptable localized discontinuities in the part. Furthermore, the use of gas may cause localized cooling of the part and die causing distortion.
Another method used to remove parts from a die is the use of vacuum cups. For example, U.S. Pat. No. 3,179,262, Material Transferring Apparatus by Carlson, et al., discloses the use of a conveyor system having vacuum cups for transferring material from one press to another. This method has several disadvantages. For example, it requires a separate system for removing the part. The vacuum cup would have to be at least as large as the part. If it were made smaller, then the problem of localized loading would appear which could cause undesirable deformation. Thus, for large parts the vacuum cup and its supports, etc., would become large and expensive. Furthermore, a specific vacuum cup would have to be fabricated for each part. This would also add to the expense.
Therefore, it is a primary object of this invention to provide an improved method of removing formed parts from a die cavity.
It is another object of this invention to provide a method of removing formed parts from a die that will minimize distortion of the part as it is removed from the die cavity.
A further object of the this invention is to provide a method for removing formed parts from a die cavity which does not require that discontinuities exist in the cavity wall.
A still further object of this invention is to provide a method of removing formed parts from a die without requiring substantial cooling of the die.